The present invention relates to a cemented plastic lens made of synthetic resin materials.
The use of plastic lenses as a substitute for glass lenses is gaining popularity today as a result of the recent improvement in plastics molding technology. Conventionally, plastic lenses have been used as single elements to take advantage of the optical properties of resin materials. However, this practice causes a problem in certain applications such as a where a plastic lens is used as binocular eyepiece; the eyepiece has a small diameter and its entrance face has a small radius of curvature, so that part of the incident rays will be totally reflected by the entrance face. In order to prevent such total reflection, the entrance face of a small curvature radius must be combined with another lens element to form a cemented lens. If one wants to compensate for chromatic aberration of the cemented plastic lens, the individual lens elements must have different optical properties, but then those elements will have different coefficients of thermal expansion.
For bonding two plastic lens elements, one may think of using adhesives that are conventionally employed in bonding glass lenses. In fact, however, the following problems have been found to occur if plastic lens elements having different coefficients of thermal expansion are bonded together by the technique commonly used in cementing glass lenses. First, plastic lenses are generally softer than glass lenses, so when two thin plastic lenses are bonded together with an adhesive, the adhesive will shrink upon hardening and an internal stress will develop in the lenses themselves to deform their shape. Secondly, if two thick plastic lenses are bonded together, not only their shape is deformed by the thermal expansion mismatch but the resulting deformation will also cause stress concentration at the interface between the cemented lens elements, which might lead to lens destruction in an extreme case.
Opthalmic lenses that are named "Combi" are also known as cemented plastic lenses. However, in such opthalmic lenses, two lens elements that are made of the same synthetic resin material but which have different curvatures (or aspheric surfaces) are bonded together in appropriate combinations. Hence, there will be no development of stress at the cemented interface due to thermal expansion mismatches, nor will there be the problem of stress development since the cemented interface has a large radius of curvature. In short, the opthalmic lens consists of cemented lens elements of the same material and, hence, is free from the aforementioned problems encountered with the cemented plastic lenses that are fabricated by bonding lens elements of dissimilar resin materials.